Radar-Based Vital Sign Estimation

ABSTRACT

In an embodiment, a method includes: receiving radar signals with a millimeter-wave radar; generating range data based on the received radar signals; detecting a target based on the range data; performing ellipse fitting on in-phase (I) and quadrature (Q) signals associated with the detected target to generate compensated I and Q signals associated with the detected target; classifying the compensated I and Q signals; when the classification of the compensated I and Q signals correspond to a first class, determining a displacement signal based on the compensated I and Q signals, and determining a vital sign based on the displacement signal; and when the classification of the compensated I and Q signals correspond to a second class, discarding the compensated I and Q signals.

TECHNICAL FIELD

The present disclosure relates generally to an electronic system and method, and, in particular embodiments, to a radar-based vital sign estimation.

BACKGROUND

Applications in the millimeter-wave frequency regime have gained significant interest in the past few years due to the rapid advancement in low cost semiconductor technologies, such as silicon germanium (SiGe) and fine geometry complementary metal-oxide semiconductor (CMOS) processes. Availability of high-speed bipolar and metal-oxide semiconductor (MOS) transistors has led to a growing demand for integrated circuits for millimeter-wave applications at e.g., 24 GHz, 60 GHz, 77 GHz, and 80 GHz and also beyond 100 GHz. Such applications include, for example, automotive radar systems and multi-gigabit communication systems.

In some radar systems, the distance between the radar and a target is determined by transmitting a frequency modulated signal, receiving a reflection of the frequency modulated signal (also referred to as the echo), and determining a distance based on a time delay and/or frequency difference between the transmission and reception of the frequency modulated signal. Accordingly, some radar systems include a transmit antenna to transmit the radio-frequency (RF) signal, and a receive antenna to receive the reflected RF signal, as well as the associated RF circuits used to generate the transmitted signal and to receive the RF signal. In some cases, multiple antennas may be used to implement directional beams using phased array techniques. A multiple-input and multiple-output (MIMO) configuration with multiple chipsets can be used to perform coherent and non-coherent signal processing as well.

SUMMARY

In accordance with an embodiment, a method includes: receiving radar signals with a millimeter-wave radar; generating range data based on the received radar signals; detecting a target based on the range data; performing ellipse fitting on in-phase (I) and quadrature (Q) signals associated with the detected target to generate compensated I and Q signals associated with the detected target; classifying the compensated I and Q signals; when the classification of the compensated I and Q signals correspond to a first class, determining a displacement signal based on the compensated I and Q signals, and determining a vital sign based on the displacement signal; and when the classification of the compensated I and Q signals correspond to a second class, discarding the compensated I and Q signals.

In accordance with an embodiment, a device includes: a millimeter-wave radar configured to transmit chirps and receive reflected chirps; and a processor configured to: generate range data based on the received reflected chirps to generate range data; detect a target based on the range data; perform ellipse fitting on in-phase (I) and quadrature (Q) signals associated with the detected target to generate compensated I and Q signals associated with the detected target; classify the compensated I and Q signals; when the classification of the compensated I and Q signals correspond to a first class, determine a displacement signal based on the compensated I and Q signals, and determine a vital sign based on the displacement signal; and when the classification of the compensated I and Q signals correspond to a second class, discard the compensated I and Q signals.

In accordance with an embodiment, a method of generating a time-domain displacement signal using a millimeter-wave radar includes: receiving reflected chirps with a receiver circuit of the millimeter-wave radar; performing a range FFT on the received reflected chirps to generate range data; detecting a target based on the range data; detecting movement of the detected target by calculating a standard deviation of the range data at a range of the detected target; when the standard deviation is below a first threshold, generating compensated in-phase (I) and quadrature (Q) signals associated with the detected target by performing ellipse fitting on I and Q signals associated with the detected target; classifying the compensated I and Q signals; when the classification of the compensated I and Q signals correspond to a first class, determining the time-domain displacement signal based on the compensated I and Q signals; and when the classification of the compensated I and Q signals correspond to a second class, discarding the compensated I and Q signals.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a radar system, according to an embodiment of the present invention;

FIG. 2 shows a flow chart of an embodiment method for generating a time-domain displacement signal from reflected radar signals, according to an embodiment of the present invention;

FIGS. 3 and 4 show exemplary in-phase (I) and quadrature (Q) plots of low quality and high quality, respectively;

FIG. 5 shows a filtered time-domain displacement signal of a heartbeat of a human target, according to an embodiment of the present invention;

FIG. 6 shows a flow chart of an embodiment method for performing the ellipse fitting step of FIG. 2, according to an embodiment of the present invention;

FIGS. 7 and 8 show flow charts of embodiment methods for performing the classification step of FIG. 2, according to embodiments of the present invention;

FIG. 9 shows a block diagram of an embodiment for classifying I-Q data using a deep neural network, according to an embodiment of the present invention; and

FIGS. 10 and 11 show block diagrams of the training of the deep neural network of FIG. 9, and of the trained deep neural network of FIG. 9, respectively, according to an embodiment of the present invention.

Corresponding numerals and symbols in different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments disclosed are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

The description below illustrates the various specific details to provide an in-depth understanding of several example embodiments according to the description. The embodiments may be obtained without one or more of the specific details, or with other methods, components, materials and the like. In other cases, known structures, materials or operations are not shown or described in detail so as not to obscure the different aspects of the embodiments. References to “an embodiment” in this description indicate that a particular configuration, structure or feature described in relation to the embodiment is included in at least one embodiment. Consequently, phrases such as “in one embodiment” that may appear at different points of the present description do not necessarily refer exactly to the same embodiment. Furthermore, specific formations, structures or features may be combined in any appropriate manner in one or more embodiments.

Embodiments of the present invention will be described in a specific context, a radar-based system and method for estimating vital signs (such as respiration rate and heartbeat rate) in a human target. Embodiments of the present invention may be used in non-human targets (such as animals), as wells as for other types of vital signs, such as heart signal shape monitoring, for example.

Embodiments of the present invention may be used in a variety of applications. For example, some embodiments may be used in patient monitoring in hospitals, sleep apnea detection. Some embodiments allow for continuous monitoring of vital signs, which may advantageously lead to better diagnosis by early detection and prevention of critical states of health.

Some embodiments may be used in applications such as presence sensing in homes and offices, driver monitoring in cars, and physiological monitoring in surveillance and earthquake rescue operations. Monitoring the vital signs of, e.g., human targets, finds wide usage in the fields of, e.g., consumer electronics, medical care, surveillance, driver assistance, and industrial applications. Some embodiments may be used in other applications.

In an embodiment of the present invention, a millimeter-wave radar is used to generate a time-domain displacement signal based on in-phase (I) and quadrature (Q) signals associated with a human target, where the time-domain displacement signal is used, e.g., to estimate a vital sign (e.g., heartbeat rate or respiration rate). Random body movements (RBM) and/or intermodulation product (IMP) that may affect the I-Q signals are compensated before the generation of the time-domain displacement signal. The compensated I-Q signals are classified, e.g., as high quality (e.g., data without RBM or IMP) or low quality (e.g., data with RBM or IMP). Low quality data are discarded and not used for estimating the vital sign. In some embodiments, a neural network is used to classify the compensated I-Q signals.

A radar, such as a millimeter-wave radar, may be used to detect and track humans. Once the human targets are identified, the radar may be used to monitor vital signs such as the heartbeat rate and/or respiration rate of the identified human target. In some embodiments, therefore, a radar, such as a millimeter-wave radar, enables a contactless, non-invasive method for vital sensing, which may advantageously increase the comfort of the human target during the vital signs monitoring.

Conventionally, radar-based vital sensing utilizes the discrete Fourier transform and determines respiration rate and heartbeat rate by detecting the maximum peaks in the frequency spectrum. Such an approach may be susceptible to multiple reflections, multipath effects, motion artifacts, RBM and IMP, for example. For example, IMP at a frequency of f_(h), f_(r) may occur between the heartbeat rate (e.g., at a frequency f_(h) between 0.7 Hz and 3 Hz) and a respiration rate (e.g., at a frequency f_(r) between 0.2 Hz and 0.5 Hz).

FIG. 1 shows radar system boo, according to an embodiment of the present invention. Radar system 100 includes millimeter-wave radar 102 and processor 104. In some embodiments, millimeter-wave radar 102 includes processor 104.

During normal operation, millimeter-wave radar 102 transmits a plurality of radiation pulses 106, such as chirps, towards scene 108 using transmitter (TX) circuit 120. In some embodiments the chirps are linear chirps (i.e., the instantaneous frequency of the chirp varies linearly with time).

The transmitted radiation pulses 106 are reflected by objects in scene 108. The reflected radiation pulses (not shown in FIG. 1), which are also referred to as the echo signal, are received by millimeter-wave radar 102 using receiver (RX) circuit 122 and processed by processor 104 to, for example, detect and track targets such as humans.

The objects in scene 108 may include static humans, such as lying human 110, humans exhibiting low and infrequent motions, such as standing human 112, and moving humans, such as running or walking humans 114 and 116. The objects in scene 108 may also include static objects (not shown), such as furniture, and periodic movement equipment. Other objects may also be present in scene 108.

Processor 104 analyses the echo data to determine the location of humans using signal processing techniques. For example, in some embodiments, a range discrete Fourier transform (DFT), such as a range fast Fourier transform (FFT), is used for estimating the range component of the location of a detected human (e.g., with respect to the location of the millimeter-wave radar). The azimuth component of the location of the detected human may be determined using angle estimation techniques.

Processor 104 may be implemented as a general purpose processor, controller or digital signal processor (DSP) that includes, for example, combinatorial circuits coupled to a memory. In some embodiments, processor 104 may be implemented with an ARM architecture, for example. In some embodiments, processor 104 may be implemented as a custom application specific integrated circuit (ASIC). Some embodiments may be implemented as a combination of hardware accelerator and software running on a DSP or general purpose micro-controller. Other implementations are also possible.

Millimeter-wave radar 102 operates as a frequency-modulated continuous-wave (FMCW) radar that includes a millimeter-wave radar sensor circuit, and one or more antenna(s). Millimeter-wave radar 102 transmits (using TX 120) and receives (using RX 122) signals in the 20 GHz to 122 GHz range via the one or more antenna(s) (not shown). Some embodiments may use frequencies outside of this range, such as frequencies between 1 GHz and 20 GHz, or frequencies between 122 GHz, and 300 GHz.

In some embodiments, the echo signals received by millimeter-wave radar 102 are filtered and amplified using band-pass filter (BPFs), low-pass filter (LPFs), mixers, low-noise amplifier (LNAs), and intermediate frequency (IF) amplifiers in ways known in the art. The echo signals are then digitized using one or more analog-to-digital converters (ADCs) for further processing. Other implementations are also possible.

Generally, monitoring a vital signs, such as heartbeat rate or respiration rate of a human target with a radar-based system is a complex endeavor. For example, the amplitude of the heartbeat signal is generally smaller than the amplitude of the respiration signal of the human target. The amplitude of the heartbeat signal is also generally smaller than the amplitude caused by the movement of the human target (e.g., when walking), as well as random body movements of the human target (e.g., such as lifting an arm, twisting the torso, etc.). Additionally, the signal shape of a single heartbeat may be dependent on the subject, the chosen measurement spot, and the distance to the antenna.

In an embodiment of the present invention, compensated I-Q signals are generated using ellipse fitting on the I-Q signals associated with the detected target. A classifier is used to classify the compensated I-Q signals, e.g., as high quality (e.g., without RBM and IMP) or low quality (e.g., with RBM or IMP). I-Q signals classified as low quality are discarded. I-Q signals classified as high quality (e.g., High Quality Data) are used to generate a time-domain displacement signal. The time-domain displacement signal is then filtered and the filtered time-domain displacement signal is used to estimate the vital sign (e.g., the heartbeat rate or respiration rate). In some embodiments, a Kalman filter is used to keep track of the estimated vital sign over time, e.g., as described in co-pending U.S. patent application Ser. No. 16/794,904, filed Feb. 19, 2020, entitled “Radar Vital Signal Tracking Using a Kalman Filter,” and incorporated herein by reference. In such embodiments, discarded I-Q signals are not used to update the Kalman filter.

In some embodiments, discarding data inflicted by RBM or IMP advantageously allows for preventing jumps in the estimated vital sign, such as in the estimated heartbeat rate or respiration rate.

FIG. 2 shows a flow chart of embodiment method 200 for generating a displacement signal from reflected radar signals, according to an embodiment of the present invention. Method 200 may be performed, e.g., by processor 104.

During step 202, millimeter-wave radar 102 transmits, e.g., linear chirps organized in frames using transmitter (TX) circuit 120. The time between chirps of a frame is generally referred to as pulse repetition time (PRT). In some embodiments, the time interval between the end of the last chirp of a frame and the start of the first chirp of the next frame is the same as the PRT so that all chirps are transmitted (and received) equidistantly.

In some embodiments, the chirps have a bandwidth of 2 GHz within the 60 GHz UWB band, the frame time has a duration of 1.28 s, and the PRT is 5 ms (corresponding to an effective sampling rate of 200 Hz).

After reflection from objects, receiver (RX) circuit 122 receives reflected chirps during step 204. During step 204, raw data are generated based on the reflected chirps received by millimeter-wave radar 102. For example, in some embodiments, during step 204, the transmitted and received radar signals are mixed to generate an IF signal. The IF signal may be referred to as the beat signal. The IF signal is then filtered (e.g., with a low-pass and/or band-pass filter) and digitized with an ADC to generate the raw data.

During step 206, a range FFT is performed on the raw data to generate range data. For example, in some embodiments, the raw data are zero-padded and the fast Fourier transform (FFT) is applied to generate the range data, which includes range information of all targets. In some embodiments, the maximum unambiguousness range for the range FFT is based on the PRT, the number of samples per chirp, chirp time, and sampling rate of the analog-to-digital converter (ADC). In some embodiments, the ADC has 12 bits. ADC's with different resolution, such as 10 bits, 14 bits, or 16 bits, for example, can also be used.

In some embodiments, the range FFT is applied on all samples of the observation window. The observation window may be implemented as consecutive windows or as sliding windows and may have a length of one or more frames. For example, in some embodiments, the observation window is implemented as a sliding window in which the length of the observation window corresponds to a plurality of time steps that are evaluated during each time step. For example, in an embodiment in which the time step is equal to 1 frame, and the observation window is a sliding window with 8 frames, then, for each frame, the last 8 frames are used as the observation window. In an embodiment, an observation window with a duration of 8 frames has a duration of about 10 s.

In some embodiments, range data, such as a range image, such as a range-Doppler image or a range cross-range image is generated during step 206.

During step 208, detection of potential targets is performed. For example, in some embodiments, an order statistics (OS) constant false alarm rate (CFAR) (OS-CFAR) detector is performed during step 208. The CFAR detector generates target detection data (also referred to as target data) in which, e.g., “ones” represent targets and “zeros” represent non-targets based, e.g., on the power levels of the range data. For example, in some embodiments, the CFAR detector performs a peak search by comparing the power levels of the range image with a threshold. Points above the threshold are labeled as targets while points below the threshold are labeled as non-targets. Although targets may be indicated by ones and non-targets may be indicated by zeros, it is understood that other values may be used to indicate targets and non-targets.

In some embodiments, targets present in the target data are clustered during step 210 to generate clustered targets (since, e.g., a human target may occupy more than one range bin). Clustering is used to “fuse” the target point cloud belonging to one target to a single target and thus determining the mean range of such single target. For example, in an embodiment, a density-based spatial clustering of applications with noise (DBSCAN) algorithm is used to associate targets to clusters during step 210. The output of DBSCAN is a grouping of the detected points into particular targets. DBSCAN is a popular unsupervised algorithm, which uses minimum points and minimum distance criteria to cluster targets, and may be implemented in any way known in the art. Other clustering algorithms may also be used. The clustered targets are used to identify the range of interest associated with the detected target (the target distance).

During step 212, movement detection of the clustered targets is performed. For example, in some embodiments, calculating the standard deviation on the complex range data at the target distance serves as movement detection. For example, in some embodiments, the complex FFT output is stored in a sliding window. Then, the amplitude of each range bin is summed up along the complete sliding window. The peak in the complete sliding window within chosen minimum and maximum ranges is the target range bin for the current frame, which serves as target detection (step 208). The standard deviation of the range bin of the detected target is indicative of the amount of movement of the target detected at the target range bin.

In some embodiments, data are discarded if the standard deviation in the target range bin along the sliding window is above a first predetermined threshold (movement detection). For example, in some embodiments, if it determined during step 214 that the human target is moving (if the standard deviation in the target range bin along the sliding window is above the first predetermined threshold), data from the current frame may be discarded. It is understood that when it is determined during step 214 that the target is not moving (when the standard deviation is below the first predetermined threshold), the target may be exhibiting some movement, such as movements of the target's hands outside the field of view of the radar, or any other movement that results in a standard deviation below the first predetermined threshold. Such may be the case, for example, of a human target that is sitting or standing, for example.

In some embodiments, data are also discarded during step 214 if the standard deviation in the target range bin along the sliding window is below a second predetermined threshold (to discard static objects such as a chair, for example).

In some embodiments, data are further processed only if, during step 214, the standard deviation in the target range bin along the sliding window is determined to be higher than a low standard deviation threshold and lower than a high standard deviation threshold.

Data not discarded during step 214 is further processed during step 216. During step 216, an ellipse fitting algorithm (also referred to as ellipse correction algorithm) is applied to the I-Q trace (of the complex range data) associated with the detected target to compensate for offset, amplitude, and gain errors. In some embodiments, the compensated data (the compensated I-Q signals) are I-Q signals that correspond to the best fit ellipse associated with the uncompensated I-Q signals. Some embodiments may avoid using the ellipse fitting algorithm, and, e.g., may use an offset compensation algorithm.

The compensated data (the compensated I-Q signals) are provided to a classifier during step 218. During step 218, the classifier estimates the quality of the compensated data and classifies the compensated data as “high quality” (first class) or “low quality” (second class). Low quality compensated data are discarded during step 220, while high quality data are further processed, e.g., in step 222. Exemplary I-Q plots of low quality (low quality data) and high quality (high quality data) are shown in FIGS. 3 and 4, respectively.

In some embodiments, RBM and IMP associated with the target results in low quality data (e.g., as shown in FIG. 3).

In some embodiments, the classifier used during step 218 is a deep learning-based classified that estimates the quality of the compensated data, e.g., using a neural network. In other embodiments, the quality of the compensated data is estimated based on, e.g., amplitude and phase imbalance values that may be generated during step 216. In other embodiments, the classifier used during step 218 is implemented using a conventional machine learning approach, such as using support-vector machines (SVM), random forest, etc.

During step 222, the angle of the compensated target data is calculated by arctangent demodulation of the I-Q signals from the selected range bin selected during step 208 (the I-Q signals associated with the detected target). The resulting phase values in the range of [−π, +π] re unwrapped between two consecutive data points during step 224. For example, during step 224, the phase is unwrapped by adding or subtracting 2π for phase jumps larger than −π or −π, respectively.

In some embodiments, steps 222 and 224 may be performed by calculating the displacement signal as

$\begin{matrix} {D = {\frac{\lambda}{4\pi} \cdot {{unwrap}\left( {\arctan\frac{Q}{I}} \right)}}} & (1) \end{matrix}$

where D represents the time-domain displacement signal, X is the wavelength of the carrier frequency, λ/2 represents the unambiguousness (phase) range, and I and Q are the in-phase and quadrature-phase components of the carrier, respectively, associated with the detected target.

During step 226, the displacement signal is filtered, and the vital sign estimation is performed during step 228. For example, in some embodiments, a band-pass FIR filter with a pass-band frequency from 0.7 Hz to 3 Hz is used, and a heartbeat frequency estimation is performed during step 228. In some embodiments, a band-pass FIR filter with a pass-band frequency from 0.2 Hz to 0.5 Hz is used, and a respiration frequency estimation is performed during step 228. In some embodiments, a fourth order Butterworth bandpass digital filter with a pass-band from 0.75 Hz to 3.33 Hz is applied to the displacement signal, e.g., for purposes of heartbeat rate estimation. In some embodiments, a fourth order Butterworth bandpass digital filter with a pass-band from 0.2 Hz to 0.33 Hz is applied to the displacement signal, e.g., for purposes of respiration rate estimation. A filter of different order and/or different type, and/or different frequencies may also be applied. In some embodiments, the pass-band of the band-pass filter is adaptive, e.g., as described in co-pending U.S. patent application Ser. No. 16/794,904, filed Feb. 19, 2020, entitled “Radar Vital Signal Tracking Using a Kalman Filter,” and incorporated herein by reference.

In some embodiments, a plurality of bandpass filters are, e.g., simultaneously applied to the time-domain displacement signal to estimate a corresponding plurality of vital signs. For example, some embodiments include a first bandpass filter for heartbeat rate estimation and a second bandpass filter for respiration rate estimation.

In some embodiments, the vital sign estimation is performed during step 228 by counting the number of peaks that the filtered time-domain displacement signal exhibits in a period of time, or by measuring a time between detected peaks of the filtered time-domain displacement signal. For example, FIG. 5 shows a filtered time-domain displacement signal of the heartbeat of a human target, according to an embodiment of the present invention. In the example of FIG. 5, the heartbeat rate is determined by counting the number of peaks in a period of time, which in this case results in a heartbeat rate estimation of 58 bpm.

FIG. 6 shows a flow chart of embodiment method 600 for performing ellipse fitting step 216, according to an embodiment of the present invention.

During step 602, the I-Q data corresponding to the clustered target are received (e.g., from step 214). The I and Q signals may be represented as

$\begin{matrix} {I = {{A_{I}{\cos\left( {\frac{4\pi\;{d(t)}}{\lambda} + \phi_{I}} \right)}} + B_{I}}} & (2) \\ {Q = {{A_{Q}{\sin\left( {\frac{4\pi\;{d(t)}}{\lambda} + \phi_{Q}} \right)}} + B_{Q}}} & (3) \end{matrix}$

where d(t) is the time-varying displacement signal, B_(I) and B_(Q) represent DC offsets, X is the wavelength of the carrier frequency.

During step 604, the amplitude and phase imbalance are estimated. For example, The amplitude imbalance A_(e) and the phase imbalance ϕ_(e) may be given by

$\begin{matrix} {A_{e} = \frac{A_{Q}}{A_{I}}} & (4) \\ {\phi_{e} = {\phi_{Q} - \phi_{I}}} & (5) \end{matrix}$

As shown, for example, in A. Singh et al., “Data-Based Quadrature Imbalance Compensation for a CW Doppler Radar System,” published in IEEE Transactions on Microwave Theory and Techniques, vol. 61, no. 4, pp. 1718-1724, April 2013, the amplitude imbalance A_(e) and the phase imbalance ϕ_(e) may be calculated by

$\begin{matrix} {A_{e} = \sqrt{\frac{1}{A}}} & (6) \\ {\phi_{e} = {\sin^{1}\left( \frac{B}{2\sqrt{A}} \right)}} & (7) \end{matrix}$

where the normalized equation of an ellipse (where I is represented in the horizontal axis and Q is represented in the vertical axis) may be given as

I ² +A×Q ² +B×IQ+C×I+D×Q+E=0  (8)

The best solution for A, B, C, D, and E may be found as

$\begin{matrix} {\begin{bmatrix} A \\ B \\ C \\ D \\ E \end{bmatrix} = {\left( {M^{T}M} \right)^{- 1}M^{T}b}} & (9) \end{matrix}$

where M and b are given by

$\begin{matrix} {M = \begin{bmatrix} Q_{1}^{2} & {I_{1}Q_{1}} & I_{1} & Q_{1} & 1 \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ Q_{N}^{2} & {I_{N}Q_{N}} & I_{N} & \ldots & 1 \end{bmatrix}} & (10) \\ {b = \begin{bmatrix} {- I_{1}^{2}} \\ \vdots \\ {- I_{N}^{2}} \end{bmatrix}} & (11) \end{matrix}$

where N is the number of (I,Q) samples.

Once Equation 9 is solved, the amplitude imbalance A_(e) and the phase imbalance ϕ_(e) may be calculated using Equations 6 and 7.

Once the amplitude imbalance A_(e) and the phase imbalance ϕ_(e) are determined, the compensated I-Q data are generated, for example, using the Gram Schmidt (GS) or ellipse fitting method in a known manner, during step 604.

FIG. 7 shows a flow chart of embodiment method 700 for performing classification steps 218 and 220, according to an embodiment of the present invention.

During step 702, an imbalance value is received (e.g., determined during step 604). In some embodiments, the imbalance value may be, e.g., the amplitude imbalance A_(e) or the phase imbalance ϕ_(e) calculated during step 604.

During step 704, the imbalance value is compared with an imbalance threshold. If the imbalance value is higher than the imbalance threshold, the compensated I-Q-data (e.g., generated during step 604) is classified as low quality. If the imbalance value is lower than the imbalance threshold, the compensated I-Q data are classified as high quality.

In some embodiments, if either of the amplitude imbalance A_(e) or the phase imbalance ϕ_(e) is higher than a respective threshold, the compensated I-Q data are classified as low quality.

FIG. 8 shows a flow chart of embodiment method 800 for performing classification steps 218 and 220, according to an embodiment of the present invention.

During step 802, compensated I-Q data are received (e.g., determined during step 604). During step 804, the compensated I-Q data are fed through a neural network. The neural network classifies the compensated I-Q data as low quality or high quality based on the neural network model.

FIG. 9 shows a block diagram of embodiment 900 for classifying I-Q data using deep neural network 904, according to an embodiment of the present invention.

As shown in FIG. 9, after reception of N chirps (e.g., 2048 chirps) from one or more consecutive frames (e.g., 8 consecutive frames), a target bin is identified (e.g., in step 208). Compensation module 902 receives I-Q data associated with the target bin (e.g., in step 602) and produces compensated I-Q data (e.g., in step 604). Deep neural network classifier 904 receives the compensated I-Q data (e.g., in step 802) and perform a multilevel classification. For example, in some embodiments, the compensated I-Q data are divided into 3 classes: RBM, IMP, and High Quality Data. Only I-Q data that is classified as “High Quality Data” is categorized as high quality (e.g., in step 220). I-Q data classified as RBM or IMP is categorized as low quality (e.g., in step 220) and discarded.

In some embodiments, classifier 904 may be implemented with one-dimensional (1D) convolutional neural network (CNN) layers. In some embodiments, the 1D CNN model advantageously learns from the I-Q time series directly.

In some embodiments, a long-short term memory (LSTM) model may be used instead of the 1D CNN model.

FIG. 10 shows a block diagram of training deep neural network 1000, according to an embodiment of the present invention. Deep neural network 904 may be trained as training deep neural network 1000. Training deep neural network 1000 includes input layer 1002, convolutional 1D layers 1004, 1010, 1018, 1026, 1032, 1038, and 1046, average pooling layers 1006, 1012, 1020, 1028, 1034, 1040, and 1048, batch normalization layers 1008, 1014, 1022, 1030, 1036, 1042, and 1050, dropout layers 1016, 1024, and 1044, and fully connected layer 1052. Different arrangements (e.g., different number of layers and/or different types of layers) may also be used.

As shown in FIG. 10, deep neural network 1000 receives batches (e.g., of 20 samples each) of training data with input layer 1002 and generates, with fully connected layer 1052, an M-element vector that corresponds to the classification of the respective data, where M is equal to or greater than 2. For example, in some embodiments, M is equal to 3, the output vector is in the form of, e.g., [“RBM” “IMP” “High Quality Data”], and the training data includes I-Q data (e.g., I-Q time series) that are pre-labeled as RBM, IMP, or High Quality Data. For example, an output vector [1 0 0] represents an “RBM” classification for the respective data; an output vector [0 1 0] represents an “IMP” classification for the respective data; and an output vector [0 0 1] represents a “High Quality Data” classification for the respective data.

In some embodiments, the output vector includes confidence values (i.e., the probability that a particular label is correct. In such embodiments, an output vector [0.8 0.15 0.05] may be interpreted as the respective data having 80% probability of corresponding to an “RBM” classification, a 15% probability of corresponding to an “IMP” classification, and a 5% probability of corresponding to a “High Quality Data” classification. In such scenario, the respective data may be assigned the classification with highest confidence (in this non-limiting example, classified as “RBM”).

In some embodiments, training data are pre-labeled as RBM when the amplitude imbalance A_(e) or the phase imbalance ϕ_(e) is higher than a respective threshold. In some embodiments, training data are pre-labeled as IMP when the difference between estimated and reference heartbeat rate is similar to the corresponding respiration rate of the human target. In some embodiments, training data are pre-labeled as High Quality Data when the criteria for RBM and IMP are not met. In some embodiments, the training data may be pre-labeled in a different manner.

During normal operation, input layer 1002 receives the training data in the form of, e.g., a 256×1 one-dimensional signals (256 data points in slow time).

The convolutional 1D layers (1004, 1010, 1018, 1026, 1032, 1038, and 1046) convolve their respective inputs and pass its results to the next layer. For example, as shown in FIG. 10, convolutional 1D layers 1004 convolves 256 slow-time data using 1024 filters; convolutional 1D layers 1010 convolves 128 slow-time data using 512 filters; convolutional 1D layers 1018 convolves 64 slow-time data using 512 filters; convolutional 1D layers 1026 convolves 32 slow-time data using 128 filters; convolutional 1D layers 1032 convolves 16 slow-time data using 128 filters; convolutional 1D layers 1038 convolves 8 slow-time data using 128 filters; and convolutional 1D layers 1046 convolves 4 slow-time data using 64 filters. Other implementations are also possible.

In some embodiments, the type of convolution filters used in the convolutional 1D layers (1004, 1010, 1018, 1026, 1032, 1038, and 1046) has a size of 3×3. Some embodiments may use other filter sizes.

Each convolutional 1D layer (1004, 1010, 1018, 1026, 1032, 1038, and 1046) uses a rectified linear unit (ReLU) as activation function. Other activation functions may also be used.

The pooling layers (1006, 1012, 1020, 1028, 1034, 1040, and 1048) smoothen the data by, e.g., applying averaging. For example, as shown in FIG. 10, pooling layer 1006 filters 256 slow-time data using 1024 filters; pooling layer 1012 filters 64 slow-time data using 512 filters; pooling layer 1020 filters 32 slow-time data using 512 filters; pooling layer 1028 filters 16 slow-time data using 128 filters; pooling layer 1034 filters 8 slow-time data using 128 filters; pooling layer 1040 filters 4 slow-time data using 128 filters; and pooling layer 1048 filters 2 slow-time data using 64 filters. Other implementations are also possible.

In some embodiments, each pooling layer has a size of 2×2 with strides 2 (the filter window is shifted by 2 units to the right and down for application of the next pooling in the neighborhood). Some embodiments may use other filter sizes and a different stride.

As shown in FIG. 10, each pooling layer (1006, 1012, 1020, 1028, 1034, 1040, and 1048) applies average pooling (using an average function). In some embodiments, a “max” function may also be used.

Batch normalization layers (1008, 1014, 1022, 1030, 1036, 1042, and 1050), subtracts the mean of the respective batch data being processed from the batch data, and the result is divided by the standard deviation of the batch data so that the batch data is normalized (so that the resulting mean is 0 and the resulting standard deviation is 1).

Dropout layers (1016, 1024, and 1044) help create redundancy in the neural network by randomly removing nodes (e.g., randomly zeroing weights on the previous convolutional layer) and corresponding edges to/from the removed nodes of the neural network. In some embodiments, 20% of the nodes are removed by each of the dropout layers. In the embodiment shown in FIG. 10, dropout layer 1016 has a spatial domain with 64 dimensions and 512 filters; dropout layer 1024 has a spatial domain with 32 dimensions and 512 filters; and dropout layer 1044 has a spatial domain with 4 dimensions and 128 filters. Other implementations are also possible.

Fully connected layer 1052 that uses, e.g., a softmax as an activation function is used to generate an M-element vector corresponding to the classification of the batch data.

During training, the generated output vector is compared with the pre-labels of the respective data batch, and the weights of the neural network are adjusted so that the classification of a respective batch of data corresponds to the respective pre-labels. The model (the neural network 1000) is refined by running a plurality of training data batches, e.g., thousands of training data batches. In some embodiments, an adaptive moment estimation (Adam) is used to optimize the deep neural network 1000.

Once trained, the trained deep neural network may be used during steps 218 and 220 to classify the compensated I-Q data. FIG. 11 shows trained deep neural network 1100, according to an embodiment of the present invention. Deep neural network 1100 may have been trained, e.g., as described in with respect to FIG. 10. Deep neural network 904 may be implemented as deep neural network 1100.

In some embodiments, a difference between the training model (e.g., training deep neural network 1000 of FIG. 10) and the inference model (e.g., trained deep neural network 1100 of FIG. 11) is the batch normalization. For example, in the training model, the batch normalization (1008, 1014, 1022, 1030, 1036, 1042, and 1050) is computed across the mini-batch used for training the network (i.e. mean and variance across the batch is computed). In the inference model, the learned global mean and variance is used (not learned) for normalization (normalization is not shown in FIG. 11).

During normal operation, input layer 1002 receives compensated I-Q data (e.g., generated during step 216). The training convolutional 1D layers and pooling layers process the received I-Q data, and fully connected layer generates a, e.g., 3-element vector that classifies the received I-Q data as, e.g., RBM, IMP, or High Quality Data. I-Q data classified as RBM or IMP is categorized as low quality (in step 220) and discarded. I-Q data classified as High Quality Data are further processed in steps 222, 224, 226, and 228.

Advantages of some embodiments include that by avoiding estimating the vital sign when the quality of the I-Q signal is low (e.g., due to RBM or IMP), jumps in the estimated vital sign (e.g., jumps in the heartbeat rate or respiration rate) may be prevented.

In some embodiments, estimating the vital sign based on a time-domain radar-based displacement advantageously allows for removal of IMP and RBM when compared with vital sign estimation methods that rely on spectral analysis and that are susceptible to corrupted estimates due to RBM and IMP, given the sensitivities of the phase in the IF signal due to minute displacements of the human target.

Example embodiments of the present invention are summarized here. Other embodiments can also be understood from the entirety of the specification and the claims filed herein.

Example 1. A method including: receiving radar signals with a millimeter-wave radar; generating range data based on the received radar signals; detecting a target based on the range data; performing ellipse fitting on in-phase (I) and quadrature (Q) signals associated with the detected target to generate compensated I and Q signals associated with the detected target; classifying the compensated I and Q signals; when the classification of the compensated I and Q signals correspond to a first class, determining a displacement signal based on the compensated I and Q signals, and determining a vital sign based on the displacement signal; and when the classification of the compensated I and Q signals correspond to a second class, discarding the compensated I and Q signals.

Example 2. The method of example 1, where classifying the compensated I and Q signals includes using a neural network.

Example 3. The method of one of examples 1 or 2, where the neural network includes a one-dimensional (1D) convolutional neural network (CNN) layer.

Example 4. The method of one of examples 1 to 3, where the neural network includes a fully-connected layer.

Example 5. The method of one of examples 1 to 4, further including training the neural network.

Example 6. The method of one of examples 1 to 5, further including: generating an amplitude imbalance value and a phase imbalance value associated with the compensated I and Q signals of the detected target; and classifying the compensated I and Q signals as the first class when the amplitude imbalance value is lower than a first threshold, and when the phase imbalance value is lower than a second threshold.

Example 7. The method of one of examples 1 to 6, further including: generating an amplitude imbalance value and a phase imbalance value associated with the compensated I and Q signals of the detected target; and classifying the compensated I and Q signals as the second class when the amplitude imbalance value is higher than a first threshold, or when the phase imbalance value is higher than a second threshold.

Example 8. The method of one of examples 1 to 7, where determining the displacement signal includes determining an angle of arrival based on the compensated I and Q signals.

Example 9. The method of one of examples 1 to 8, where determining the vital sign includes filtering the displacement signal, and estimating heartbeat rate or respiration rate based on the filtered displacement signal.

Example 10. The method of one of examples 1 to 9, where determining the vital sign includes using a Kalman filter to track changes of the vital sign over time.

Example 11. The method of one of examples 1 to 10, where discarding the compensated I and Q signals includes avoiding updating the Kalman filter with the compensated I and Q signals.

Example 12. The method of one of examples 1 to 11, where detecting the target includes performing a peak search based on the range data.

Example 13. The method of one of examples 1 to 12, where generating the range data includes performing a range FFT on the received radar signals.

Example 14. The method of one of examples 1 to 13, further including transmitting radar signals, where the received radar signals are based on the transmitted radar signals.

Example 15. A device including: a millimeter-wave radar configured to transmit chirps and receive reflected chirps; and a processor configured to: generate range data based on the received reflected chirps to generate range data; detect a target based on the range data; perform ellipse fitting on in-phase (I) and quadrature (Q) signals associated with the detected target to generate compensated I and Q signals associated with the detected target; classify the compensated I and Q signals; when the classification of the compensated I and Q signals correspond to a first class, determine a displacement signal based on the compensated I and Q signals, and determine a vital sign based on the displacement signal; and when the classification of the compensated I and Q signals correspond to a second class, discard the compensated I and Q signals.

Example 16. The device of example 15, where the processor is configured to classify the compensated I and Q signals using a neural network that includes a one-dimensional (1D) convolutional neural network (CNN) layer.

Example 17. The device of one of examples 15 or 16, where the processor is configured to determine the displacement signal as

${D = {\frac{\lambda}{4\pi} \cdot {{unwrap}\left( {\arctan\frac{Q}{I}} \right)}}},$

where λ is a wavelength of a carrier frequency of the transmitted chirps, and where I and Q are the in-phase and quadrature-phase components, respectively, associated with the detected target.

Example 18. A method of generating a time-domain displacement signal using a millimeter-wave radar, the method including: receiving reflected chirps with a receiver circuit of the millimeter-wave radar; performing a range FFT on the received reflected chirps to generate range data; detecting a target based on the range data; detecting movement of the detected target by calculating a standard deviation of the range data at a range of the detected target; when the standard deviation is below a first threshold, generating compensated in-phase (I) and quadrature (Q) signals associated with the detected target by performing ellipse fitting on I and Q signals associated with the detected target; classifying the compensated I and Q signals; when the classification of the compensated I and Q signals correspond to a first class, determining the time-domain displacement signal based on the compensated I and Q signals; and when the classification of the compensated I and Q signals correspond to a second class, discarding the compensated I and Q signals.

Example 19. The method of example 18, where generating the compensated I and Q signals includes using a Gram Schmidt (GS) or ellipse correction method.

Example 20. The method of one of examples 18 or 19, where classifying the compensated I and Q signals includes using a neural network that includes a one-dimensional (1D) convolutional neural network (CNN) layer.

While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments. 

What is claimed is:
 1. A method comprising: receiving radar signals with a millimeter-wave radar; generating range data based on the received radar signals; detecting a target based on the range data; performing ellipse fitting on in-phase (I) and quadrature (Q) signals associated with the detected target to generate compensated I and Q signals associated with the detected target; classifying the compensated I and Q signals; when the classification of the compensated I and Q signals correspond to a first class, determining a displacement signal based on the compensated I and Q signals, and determining a vital sign based on the displacement signal; and when the classification of the compensated I and Q signals correspond to a second class, discarding the compensated I and Q signals.
 2. The method of claim 1, wherein classifying the compensated I and Q signals comprises using a neural network.
 3. The method of claim 2, wherein the neural network comprises a one-dimensional (1D) convolutional neural network (CNN) layer.
 4. The method of claim 2, wherein the neural network comprises a fully-connected layer.
 5. The method of claim 2, further comprising training the neural network.
 6. The method of claim 1, further comprising: generating an amplitude imbalance value and a phase imbalance value associated with the compensated I and Q signals of the detected target; and classifying the compensated I and Q signals as the first class when the amplitude imbalance value is lower than a first threshold, and when the phase imbalance value is lower than a second threshold.
 7. The method of claim 1, further comprising: generating an amplitude imbalance value and a phase imbalance value associated with the compensated I and Q signals of the detected target; and classifying the compensated I and Q signals as the second class when the amplitude imbalance value is higher than a first threshold, or when the phase imbalance value is higher than a second threshold.
 8. The method of claim 1, wherein determining the displacement signal comprises determining an angle of arrival based on the compensated I and Q signals.
 9. The method of claim 1, wherein determining the vital sign comprises filtering the displacement signal, and estimating heartbeat rate or respiration rate based on the filtered displacement signal.
 10. The method of claim 1, wherein determining the vital sign comprises using a Kalman filter to track changes of the vital sign over time.
 11. The method of claim 10, wherein discarding the compensated I and Q signals comprises avoiding updating the Kalman filter with the compensated I and Q signals.
 12. The method of claim 1, wherein detecting the target comprises performing a peak search based on the range data.
 13. The method of claim 1, wherein generating the range data comprises performing a range FFT on the received radar signals.
 14. The method of claim 1, further comprising transmitting radar signals, where the received radar signals are based on the transmitted radar signals.
 15. A device comprising: a millimeter-wave radar configured to transmit chirps and receive reflected chirps; and a processor configured to: generate range data based on the received reflected chirps to generate range data; detect a target based on the range data; perform ellipse fitting on in-phase (I) and quadrature (Q) signals associated with the detected target to generate compensated I and Q signals associated with the detected target; classify the compensated I and Q signals; when the classification of the compensated I and Q signals correspond to a first class, determine a displacement signal based on the compensated I and Q signals, and determine a vital sign based on the displacement signal; and when the classification of the compensated I and Q signals correspond to a second class, discard the compensated I and Q signals.
 16. The device of claim 15, wherein the processor is configured to classify the compensated I and Q signals using a neural network that comprises a one-dimensional (1D) convolutional neural network (CNN) layer.
 17. The device of claim 15, wherein the processor is configured to determine the displacement signal as ${D = {\frac{\lambda}{4\pi} \cdot {{unwrap}\left( {\arctan\frac{Q}{I}} \right)}}},$ wherein λ is a wavelength of a carrier frequency of the transmitted chirps, and wherein I and Q are the in-phase and quadrature-phase components, respectively, associated with the detected target.
 18. A method of generating a time-domain displacement signal using a millimeter-wave radar, the method comprising: receiving reflected chirps with a receiver circuit of the millimeter-wave radar; performing a range FFT on the received reflected chirps to generate range data; detecting a target based on the range data; detecting movement of the detected target by calculating a standard deviation of the range data at a range of the detected target; when the standard deviation is below a first threshold, generating compensated in-phase (I) and quadrature (Q) signals associated with the detected target by performing ellipse fitting on I and Q signals associated with the detected target; classifying the compensated I and Q signals; when the classification of the compensated I and Q signals correspond to a first class, determining the time-domain displacement signal based on the compensated I and Q signals; and when the classification of the compensated I and Q signals correspond to a second class, discarding the compensated I and Q signals.
 19. The method of claim 18, wherein generating the compensated I and Q signals comprises using a Gram Schmidt (GS) or ellipse correction method.
 20. The method of claim 18, wherein classifying the compensated I and Q signals comprises using a neural network that comprises a one-dimensional (1D) convolutional neural network (CNN) layer. 